EOR Design Drawings for Pre-Engineered Roof Trusses

[JennyNakamura]

Hi Everyone!

As the EOR for a new building with pre-engineered roof trusses (wood or cold-formed steel), do you show a roof plan with C&C pressures and zones for the truss designer/fabricator? Or, do you just state the required wind design parameters on the notes (Kzt, Kd, V, exposure, etc.) and let the truss fabricators calculate wind loads themselves? Will fabricators even calc wind loads on their own if you show them on your drawings as an independent back check, since they typically will stamp their own shop drawings, or do they just take the EOR's word for it? Is it a regional thing like with steel connection details? What the "right" thing for the EOR to do when putting together design drawings for bidding purposes?

 

[Ron]

State the parameters and let the delegate engineer develop the pressures. Then review for conformance with intent.

 

[msquared48]

I second the motion.

Mike McCann
MMC Engineering

 

[woodman88]

You need to calculate the building required C&C connections for the truss to wall. Than check that the truss designs meet or exceed your calculated uplifts. You should also be checking that the gravity load case for the truss meets your calculated loads. I have typically required that the trusses be design for a 300# future mechanical load placed anywhere along the top and bottom chords. I had some not include the loads. Others had design the truss for the 300# load but not concurrent with the roof live loads. Rather then looking at their calculations of loading, I find that just checking the reactions is the quickest method of checking the truss drawing for complacent with the building requirements.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ATSE]

When two different engineers communicate, nothing is better than a load diagram right on the dwgs, showing the critical load combinations as defined by ASCE 7.
Calculating the loads is not much effort, and you have the opportunity to add loads for likely (but not certain) future mechanical loads that the truss engineer will not add. 3 psf extra can provide a lot of extra capacity for a minor cost increase.
If the truss engineer calculates loads (wind, seismic, dead ... for vertical up and down, and lateral at the wall...) that are different than what you expect, all of the engineering effort (including your review and coordination time) after that is wasted.
In addition, I would be explicit about the connection of their component to your superstructure, since the coupling and rigidity ("fixity") of the connection can load your structure in ways you do not intent.
Even for reasons of pure self-interest, the more you give the "downstream" engineer, the less back-and-forth there will be.

 

[ron9876]

Check your stare requirements.

 

[woodman88]

ATSE - Truss engineers (in the wood trusses and light gage metal trusses) typically never see the building plans. Some "designer" at the truss manufacturer plant takes the information off the plans, runs a truss design, than sends the input information to the truss engineer. The truss program kicks out the designs for the truss engineer to review and seal. The wind input for most of the truss programs will be only what is required by the building code to be on the plans. The IBC (2006 section 1603.1.4) only requires the basic wind speed, wind importance factor, internal pressure coefficient and the C&C wind pressure for external component design.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JennyNakamura]

Thanks everyone for the great answers!

Just to clarify, I always show the truss diagrams with dead and live load requirements. I also always calculate the reactions, both lateral and vertical (uplift) based on my own wind calculations, mostly for comparison with the truss designers reactions as woodman88 pointed out.

I was just curious how others handle the C&C wind uplift when dealing with delegated truss designers, as I have seen both ways (defining only wind parameters on the drawings and showing roof plans with C&C pressures and Zones).

 

[nuche1973]

We've supplied the load parameters and any special point loads. However, everytime the shop drawings are reviewed the loads are usually incorrect. Be cognizant of the following: Top chord load, overhangs, uplift, snow drift and the disclaimer that accompanies the seal.

There are days when I wake up feeling like the dumbest man on the planet, then there are days when I confirm it.

 

[a2mfk]

Though I agree with Ron, one could make the argument that you should provide a pressure diagram since its a pre-engineered component. We do this for steel joist roofs, but not usually for pre-eng wood or lt gage trusses. As has been mentioned, the truss mfr has all of the wind pressure calculations built into their truss design program so you'd be doing the pressure diagram as an exercise, though some building departments may require this.

We design all the connections conservatively up front for the assumed truss layout. But we gave up a long time ago trying to get the truss manufacturer to lay out the framing plan a specific way, and also gave up guessing. Also I have come to the conclusion that they know trusses and have the programs to quickly come up with the most efficient design. So for CYA, we put a large note on the roof framing plan indicating the truss connections are preliminary, and final design will be based on the final truss layout and reactions provided on the truss shop dwgs. We also reserve the right to modify the truss connections as needed. The cost implications of connection modifications should be minimal anyway.

Finally, remember to design the connections for all forces you are imparting on them, not just uplift, but possibly out-of-plane lateral forces from the walls, and shear forces from the diaphragm if you do not use blocking. Simpson provides all of these allowable reactions in their catalogs with diagrams so it is pretty clear, and then you just use their interaction equations.

 

[ATSE]

Woodman - while your statement is probably true at some facilities (I have had the most trouble with pre-engineered wood truss manufacturers), trusses - steel or wood - cannot be designed correctly without stated axial chord force demands that are simultaneous with bending demands.
Simple loading on a component can be described in paragraph form, but for typical load conditions with mechanical equipment on the top chord, hanging loads on the bottom chord, wind uplift, and chord axial loads from wind/seismic, and so on, a picture is worth a thousand words.

 

[msquared48]

ron9876:

What are the requirements in your stare?

Mike McCann
MMC Engineering

 

[jhenry]

If you are in a high wind load or seismic area, then there are forces tranmitted from the walls to the trusses. These forces need to shown on the drawings for the truss mfgr to account for

 

[ron9876]

Florida requires that you spell out the actual loads required for the wood truss design. South Florida (Broward and Dade Counties) also requires that you put the horizontal and vertical truss reactions and your planned connector capacities on your drawings.

 

[trainguy]

Guys,

Wouldn't it be great if all posters from the United Stares could proofread their posts...

tg